1. Field of the Invention
The present invention relates to a novel material useful as a catalyst support, a catalyst, and an adsorbent and a method for preparing same. The invention particularly relates to activated carbon and phosphorus-based compounds which are mixed with or impregnated throughout the carbon and then heat-treated to form novel carbon compositions. The invention also relates to use of the carbon-based composition in a wide range of catalytic applications, particularly as a substrate for supporting one or more metal catalytic compounds. Additionally, the novel composition alone is anticipated to be useful as a catalyst or as an adsorbent material.
2. Description of Prior Art
Activated carbon is a well-established material for use as a catalyst support or as a catalyst in its own right. As a support, its network of internal pores provides a high surface area accessible for the dispersion of catalytically active agents. Commercial examples include carbon-based noble metal catalysts for hydrogenation and carbon-supported mercuric chloride catalysts for production of vinyl chloride. Aside from performance, a practical advantage with activated carbon, compared to other catalyst supports such as ceramic materials (silicon dioxide, aluminum oxide, etc.), is the ability to burn off the carbon support as a means of recovering high-value catalyst metals and minimizing a solid waste disposal problem.
In large-scale catalytic petroleum refining processes, however, ceramic catalyst supports are predominantly utilized as in the case of commercially available hydroprocessing catalysts, for example, most of which use aluminum oxide (alumina) supports. It is known in the catalyst support art that phosphorus can be added in alumina-based systems as a promotor or modifier of catalytic properties. U.S. Pat. No. 3,969,273, for example, teaches a method for mixing a solubilized phosphorus-containing compound such as phosphoric acid or ammonium phosphate with powdered alumina, extruding the mixture, and calcining to produce catalyst supports containing between 1 and 10% phosphate. U.S. Pat. No. 4,629,717 discloses a novel phosphorus-containing alumina produced by incorporating a phosphorus-containing compound into an alumina hydrosol mixture, gelling the admixture, drying, and calcining to produce catalyst supports containing a molar ratio of phosphorus to aluminum of from 1:1 to 1:100.
Phosphorus also can be incorporated in ceramic-based catalysts during the addition of catalytic agents, as in the case of U.S. Pat. No. 4,810,687 where a solubilized mixture of one or more metal salts and a phosphorus-containing compound are added to alumina hydrogel. The mixture is extruded, dried, and then calcined to produce the catalyst. The catalyst is reported as useful for hydroprocessing and other applications.
The utility of activated carbon as an alternative catalyst support for catalytic refining of petroleum is disclosed in U.S. Pat. No. 3,997,473. A cobalt-molybdenum dispersion on activated carbon containing about 580 m.sup.2 /g surface area is described to exhibit high hydrodesulfurization activity towards petroleum residues. Unlike the present invention, this catalyst support and resultant catalyst do not contain a phosphorus-based additive to modify catalytic performance.
U.S. Pat. No. 2,580,647 discloses a method for producing a phosphate-laden carbon for catalytic applications in which a high concentration of phosphate, in excess of 25% by weight, is left imbedded within a carbon char as a consequence of heating vegetable-matter and phosphoric acid to between 200.degree. and 650.degree. C. As appreciated by those skilled in the art, such a process invariably results in a material with little available surface area since the chemical activant (phosphoric acid) is not removed from the internal pores of the char by washing or by other means. The present invention, on the other hand, teaches a method whereby most of the original high surface area of the activated carbon starting material is retained, as a consequence of impregnating the activated carbon with a controlled amount of at least one phosphorus-containing compound, followed by heat-treatment.
Some have indicated that added phosphate actually poisons carbon supported Co--Mo or Ni--Mo catalysts for hydrodesulfurization. (See Bouwens et al., "The Influence of Phosphorus on the Structure and Hydrodesulfurization Activity of Sulfided Co and Co--Mo Catalysts Supported on Carbon and Alumina," J. Catal., Vol. 128, 559-568 (1991) and Eijshouts et al., "The Effect of Phosphorus on the Quinoline Hydrodenitrogenation Activity of Carbon-Supported Mo, Ni, and Ni--Mo Sulfide Catalysts," Proceedings of the Nineteenth Biennial Conference on Carbon June 25-30, 1989, 46-47 (1989).)
There is a continuing need in the field of hydroprocessing for catalysts which exhibit improvements in hydrogenation, hydrodesulfurization, dehalogenation, etc. The implementation of stricter environmental regulations also results in a need for research to find catalysts which provide more efficient performance.
There does not appear to be any art which teaches or suggests that carbon substances of a particular surface area could be treated with phosphorus-based compounds and heat treated to result in a carbon composition containing phosphorus in an amount above 2.5% by weight of the resultant composition, and possessing most of the original high surface area of the activated carbon starting material. Neither is it known that such a carbon composition would provide desirable improvements when used alone or as a support for hydroprocessing catalysts.